Flexible low-power source-gated transistors with solution-processed metal–oxide semiconductors

Dingwei Li a, Momo Zhao b, Kun Liang a, Huihui Ren a, Quantan Wu c, Hong Wang *b and Bowen Zhu *a
aKey Laboratory of 3D Micro/Nano Fabrication and Characterization of Zhejiang Province, School of Engineering, Westlake University, Hangzhou 310024, China. E-mail: zhubowen@westlake.edu.cn
bKey Laboratory of Wide Band Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi'an 710071, China. E-mail: hongwang@xidian.edu.cn
cKey Laboratory of Microelectronic Devices and Integrated Technology, Institute of Microelectronics of Chinese Academy of Sciences, Beijing 100029, China

Received 26th August 2020 , Accepted 3rd October 2020

First published on 6th October 2020


Abstract

Source-gated transistors (SGTs) with Schottky barriers have emerged as extraordinary candidates for constructing low-power electronics by virtue of device simplicity, high gain, and low operation voltages. In this work, we demonstrate flexible low-power SGTs with solution processed In2O3 channels and Al2O3 gate dielectrics on ultrathin polymer substrates, exhibiting light area density (0.56 mg cm−2), low subthreshold swing (102 mV dec−1), low operation voltage (<2 V), fast saturation behaviors (0.2 V), and low power consumption (46.3 μW cm−2). These achievements pave the way for employing the unconventional SGTs in wearable applications where low-power dissipation and high mechanical flexibility are essential.


Rapid advances in the Internet of Things demand thin-film electronics with high electrical and mechanical performance, large-area process capability, low-cost, as well as low-power dissipation.1–3 Metal–oxide semiconductors (MOSs) have been highly recognized as promising candidates in constructing high-performance thin film transistors (TFTs) for large-area electronics including active-matrix displays, flexible sensor arrays, and wearable devices.4–7 Oxide semiconductors exhibit high optical transparency, good uniformity, and high electrical performance and are capable of being fabricated via low-temperature and non-vacuum solution-based processes, holding great promises for emerging applications in flexible electronics where low cost and process flexibility are essential.8–13 Despite these advantages, the advances in raising the multifunctionality and complexity of flexible integrated electronics highly demand low-power TFTs, because the supply voltage and lifetime of portable batteries are limited.14–16 Nevertheless, solution-processed metal oxide TFTs are often depletion-mode devices with severe negative threshold voltages (Vth) due to the heavily n-doped semiconductor films, resulting in high power consumption.

Source-gated transistors (SGTs) have emerged as intriguing device configurations to achieve high-gain low-power transistors by replacing conventional ohmic source electrodes with diode-like Schottky contacts.17–19 SGTs can mitigate the negatively shifted Vth and suppress leakage current in solution-processed TFTs by forming a reverse-biased Schottky barrier at the metal–semiconductor interface, providing effective electron confinement and enhancement-mode operation.20–22 Moreover, SGTs saturate at lower voltages compared with conventional ohmic devices, enabling low-operation voltage and low-power dissipation.23–25 Although metal oxide SGTs have been prepared via vacuum-based fabrication techniques, their rigid nature and high-temperature processes limit their applications in flexible and wearable electronics. Recently, flexible SGTs based on organic semiconductors (OSCs) have been developed but their electrical performance is limited by the low mobility of organic materials.23 Therefore, it is imperative to develop advanced oxide SGTs that take full advantage of the inexpensive solution-processes, outstanding electrical performance, and high mechanical flexibility, paving the way for future low-power deformable electronic devices and circuits.

In this work, we achieved flexible Schottky barrier SGTs based on solution-processed indium oxide (In2O3) semiconductors. The In2O3 SGTs are fabricated by utilizing asymmetrical source/drain (S/D) electrodes, where Al is used as an ohmic contact (drain) and Au as the Schottky contact (source). The devices show lower voltage saturation performance and higher gain compared with conventional ohmic In2O3 TFTs. Furthermore, we demonstrated flexible light weight In2O3 SGTs using solution-processed high-k Al2O3 as a gate dielectric on ultrathin (∼1.7 μm thick) polyimide substrates. The flexible SGTs exhibit outstanding mechanical and electrical performance, with a light area density of 0.56 mg cm−2, low operation voltage of <2 V, fast saturation behaviors (0.2 V), low subthreshold swing (102 mV dec−1), and low power consumption (46.3 μW cm−2). These achievements open opportunities for applying metal oxide SGTs in soft and wearable interfaces that require low-power dissipation and excellent mechanical flexibility.

We fabricated In2O3 SGTs with asymmetric S/D electrodes in a bottom-gate, top-contact (BGTC) configuration on an Si substrate with SiO2 dielectric, as illustrated in Fig. 1a. The In2O3 channels were prepared via sol–gel chemistry by spin-coating precursor solutions, followed by annealing in air (see Experimental section). The In2O3 channel patterns were enabled by deep-ultraviolet (DUV) assisted direct-light-patterning techniques as reported before, where In2O3 precursors directly served as negative “photoresist” and became solidified after exothermic combustion reactions.26 This generates In2O3 films with ultrathin thickness (∼9.4 nm), as illustrated in the height profile from the atomic force microscopy (AFM) image (Fig. 1b). Moreover, the In2O3 film depicts a small surface roughness of only 0.5 nm (Fig. 1c), which is critical for suppressing surface scattering and charge trapping.


image file: d0nr06177h-f1.tif
Fig. 1 Solution-processed In2O3-based source-gated transistor with asymmetric source/drain electrodes. (a) Schematic showing the device structure of In2O3 SGT. (b) AFM height profile (top) and corresponding AFM image (bottom) of as-prepared In2O3 thin film of thickness of ∼9.4 nm. (c) AFM image of In2O3 film showing a surface roughness of 0.5 nm over an area of 2 × 2 μm. (d) UPS spectrum of solution-processed In2O3 thin film. The secondary electron cut-off and VBM are determined by the linear extrapolation of the leading edges of low-energy photons and high-energy electrons, respectively. (e) Energy diagram of In2O3 SGT. (f) Drain current as a function of drain voltage at a gate bias of 0 V for ohmic TFT and SGT.

To investigate the materials properties and valence band structure of In2O3, spectroscopy analysis using ultraviolet-visible spectroscopy (UV-vis) and ultraviolet photoelectron spectroscopy (UPS) was performed. The UV-vis transmission spectrum and Tauc analysis of In2O3 on quartz substrate are illustrated in Fig. S1. The In2O3 film exhibits a wide optical bandgap of 3.62 eV, providing high transparency in the visible spectrum. From the UPS spectrum (Fig. 1d), the conduction band minimum (CBM) of −3.90 eV and valence band maximum (VBM) of −7.52 eV can be determined for In2O3. Combining the results with UV-vis and UPS spectra, the energy band diagram of In2O3 with metals of different work functions can be constructed. Fig. 1e illustrates the band diagram of In2O3 with a high work function metal, Au (5.10 eV),27,28 and low work function metal, Al (4.15 eV).23,29 The large gap between the electron affinity of In2O3 (3.90 eV) and work function of the Au electrode results in a 1.20 eV Schottky barrier, while In2O3 keeps an ohmic contact with Al. To validate the different contact behaviors between Au and Al, we fabricated transistors with S/D electrodes of both symmetric Al/Al contacts and asymmetric Au/Al contacts. As shown in Fig. 1f, the source–drain current (IDS) exhibited a linear relationship with voltage (VDS), indicating good ohmic contacts with Al/Al electrodes. In contrast, rectifying contacts were obtained in asymmetric Au/Al electrodes, confirming the Schottky barrier between In2O3 and Au, where only charge carriers under high electric field can reach the drain electrodes. To further confirm the Schottky barrier, we also fabricated a diode with a vertical structure of Au/In2O3/Si (p++). As shown in Fig. S2a, the diode exhibits a typical Schottky contact behavior with a rectification ratio of ∼103.

The Schottky barrier between In2O3 and Au facilitated the construction of source-gated transistors (SGTs). An optical image of the In2O3 SGTs with asymmetric Au/Al S/D electrodes is presented in Fig. 2a. As schematically illustrated in Fig. 2b, a small drain bias can deplete semiconductors at the source edge by forming a depletion envelope beneath the source electrode. In this way, SGTs can saturate at low drain voltage with flat output characteristics, which is beneficial for high-gain low-power electronics.25


image file: d0nr06177h-f2.tif
Fig. 2 Electrical performance of ohmic TFTs and SGTs. (a) An image of an SGT device. (b) Schematic showing the source pinch-off state of the SGT. (c) Output characteristics of ohmic TFTs. (d) Output characteristics of Schottky-barrier SGTs. (e) Transfer curves of ohmic TFTs and SGTs. (f) Intrinsic gains of ohmic TFTs and SGTs as a function of drain voltage.

The output characteristics of ohmic TFTs with Al/Al S/D electrodes and SGTs with Au/Al S/D electrodes are depicted in Fig. 2c and d, respectively. The ohmic TFT shows typical pinch-off behaviors in line with the gradual channel approximation model, exhibiting a high saturation voltage (VDS_Sat) of 28 V at VGS = 20 V. In comparison, the SGT shows distinctive lower VDS_Sat (12 V at VGS = 20 V), together with higher and more stable output impedance due to the reverse-biased Schottky barrier. The SGT saturates faster because the current is primarily modulated by the effective barrier height of the Schottky barrier and it saturates when the Schottky source is depleted—prior to when the whole semiconductor channel is depleted of charge.21

To quantify the influence of the Schottky barrier on the saturation behaviors of TFTs, we investigated the relationship between VDS_Sat and VGS, which can be expressed by the following equation:30

 
image file: d0nr06177h-t1.tif(1)
where Vth is the threshold voltage of the parasitic thin film transistor, Cd and Cox are the capacitance per unit area of the semiconductor and gate dielectric (34.5 nF cm−2, εr = 3.9 for SiO2), respectively, and K is a constant which represents the drain voltage required to deplete the charge at the semiconductor/insulator interface. Cd was extracted as 52.4 nF cm−2 from the capacitance–voltage (CV) measurements of In2O3 with a metal–semiconductor–metal structure (Fig. S2b). The carrier concentration of In2O3 is extracted to be 1.15 × 1017 cm−3, from CV measurement (Fig. S2c).

As illustrated in Fig. S3, the SGTs exhibit much lower VDS_Sat values than that of ohmic TFTs. Moreover, VDS_Sat is proportional to VGS with a lower slope of 0.2 in SGTs, compared to that of ∼1 in ohmic TFTs. The discrepancy can be attributed to the Schottky barrier in SGTs, where a depletion envelope will form before the channel is fully depleted. The fast saturation behaviors of SGTs contribute to low power dissipation. The power consumption (Psat = IDS × VDS/(W × L)) of SGTs was calculated as 16 mW cm−2, which was two orders of magnitude lower than that of the ohmic counterpart (4.5 W cm−2), demonstrating the advantages of SGTs for electronics that require low-power consumption.

In addition to faster saturation, SGTs can provide better switching performance due to the Schottky barrier in Au/In2O3. The representative transfer curves (IDSVGS) of ohmic TFTs and SGTs are presented in Fig. 2e. In addition, Fig. S4 presents the transfer curves of 20 devices for both ohmic TFTs and SGTs. The ohmic TFTs show a severe negative threshold voltage (Vth) of −18 ± 2.7 V due to the high carrier concentration in solution-processed highly doped In2O3. However, the incorporation of Schottky contact with In2O3 can provide better switching properties. As a result, the SGTs possess Vth of 2.1 ± 1.9 V, indicating that enhancement-mode devices can be effectively achieved by employing source-gated device configuration in highly doped n-type transistors. Moreover, SGT shows a lower subthreshold swing (SS) of 402 ± 67 mV dec−1 compared to 473 ± 176 mV dec−1 in ohmic TFTs. At the same time, the Schottky barrier in SGTs contributes to a lower off current, but a lower current on/off ratio (Ion/off = 3.7 × 105) compared with ohmic TFTs (Ion/off = 3.0 × 106) due to the lower on current. The electron mobilities μFE calculated for SGTs and ohmic TFTs are 1.7 ± 1.5 and 5.7 ± 1.2 cm2 V−1 s−1, respectively. It should be noted that the lower μFE in SGT should not be regarded as the true mobility of In2O3 but originated from the Schottky contacts in SGTs.24

The low saturation voltage and high output impedance in SGTs result in a high intrinsic gain, which is a critical figure of merit for transistors.31Fig. 2f depicts the intrinsic gains of ohmic TFTs and SGTs, extracted by the ratio of transconductance (gm = ∂IDS/∂VGS) to output conductance (gd = ∂IDS/∂VDS). Using linear fittings, the SGT shows a high intrinsic gain of 792 at VGS = 20 V, almost forty times higher than that of the ohmic device (intrinsic gain = 20). The higher intrinsic gains of SGTs are promising for improving the accuracy of wearable sensors by amplifying weak signals obtained at skin interfaces.32

To study the influence of channel thickness on device performance, we fabricated SGTs by spin-coating In2O3 films with different layers (1, 3, and 5 layers). The thicknesses of In2O3 films with 3 and 5 layers are ∼15.8 nm and ∼18.2 nm, respectively (Fig. S5a and b). Corresponding transfer curves and output characteristics are illustrated in Fig. S5c and d. With increased channel thickness (from 1 to 5 layers), a negative shift in Vth (∼2 V) and elevated output current were observed, which could be attributed to the larger number of charge carriers in thicker In2O3 films.

To further explore the advantages of SGTs in electronic circuits, we constructed logic inverters based on In2O3 SGTs and used a 2 MΩ resistor as a load. The corresponding circuit diagram is illustrated in Fig. 3a. Fig. 3b and c exhibit the performance of inverters based on ohmic TFTs and SGTs at different supply voltages (VDD), respectively. The SGT inverter exhibits a maximum gain (gainmax = max(−∂Vout/∂Vin)) of 17 at VDD = 30 V, doubling the performance of ohmic inverter (gainmax = 8) at the same supply voltage.


image file: d0nr06177h-f3.tif
Fig. 3 Inverter performance based on ohmic TFTs and SGTs. (a) Illustration of the inverter circuit. (b–c) Input–output characteristics and the corresponding voltage gains of ohmic TFTs (b) and SGTs (c).

The superior performance of SGTs renders them feasible for flexible electronics applications. However, flexible SGTs remain scarce due to the challenges in achieving high-performance electronic materials, especially in solution-based processes. A comparison among TFTs with Schottky barriers is presented in Table S1. Most of the current SGTs are based on rigid substrates.

To demonstrate the feasibility of SGTs in flexible electronics, we fabricated In2O3 SGTs on ultrathin PI films (∼1.7 μm thick, Fig. S6) with solution-processed high-k Al2O3 as the gate dielectric. The flexible device configuration is illustrated in Fig. 4a. The whole device is so light in weight (0.56 mg cm−2) that it can be sustained by the stamens of Hypericum monogynum (Fig. 4b). In comparison, conventional devices on silicon wafer hold >100 times heavier weight (81.6 mg cm−2). The transfer curves of flexible ohmic TFTs and SGTs are illustrated in Fig. 4c. In addition, we measured the gate capacitance using a metal/insulator/metal capacitor structure. As illustrated in Fig. S7, the Al2O3 dielectric layer exhibits a high areal capacitance of 183.0 nF cm−2 and a high breakdown voltage of ∼3.5 MV cm−1. As a result, the flexible In2O3 TFTs and SGTs with Al2O3 dielectric exhibit field-effect mobility (μFE) of 15.6 and 9.0 cm2 V−1 s−1, respectively.


image file: d0nr06177h-f4.tif
Fig. 4 Electrical characteristics of flexible SGTs and ohmic TFTs on PI substrates. (a) Schematic showing the structure of flexible SGT device. (b) A digital photo of flexible SGT sustained by the stamens of Hypericum monogynum, showing ultralightweight. (c) Transfer curves of flexible ohmic TFT and SGT. The SGT shows a much lower SS of 102 mV dec−1 compared to 308 mV dec−1 of ohmic TFT. (d–e) Output characteristics of flexible ohmic TFTs (d) and SGT (e). (f) Intrinsic gains of ohmic TFTs and SGTs as a function of drain voltage.

Fig. 4d and e depict the output characteristics of flexible ohmic TFTs (Fig. 4d) and SGTs (Fig. 4e) based on the Al2O3 dielectric, respectively. Still, the flexible SGT shows a much lower saturation voltage (VDS_Sat = 0.2 V at VGS = 1 V) and a larger output impedance, resulting in an even lower power consumption (Psat = 46.3 μW cm−2) than its ohmic counterpart (29.5 mW cm−2). The intrinsic gain was calculated as ∼1000, more than two orders of magnitude than ohmic devices (∼10) (Fig. 4f). Similar to devices with the SiO2 dielectric, VDS_Sat is proportional to VGS with a lower slope of ∼0.17 in flexible SGTs, while that in flexible ohmic TFTs is ∼1.30 Importantly, the flexible SGTs outperform ohmic TFTs in terms of subthreshold swing, off current, on/off ratio, and Vth, as illustrated in Fig. 5 and Table 1, unveiling the advantages of SGTs in improving the electrical performance of solution-processed devices.


image file: d0nr06177h-f5.tif
Fig. 5 Comparison of device performance between flexible ohmic TFTs and SGTs fabricated on PI substrates. The SGTs exhibit better electrical performance with (a) SS, (b) current on/off ratio, and (c) Vth, demonstrating the advantages of SGTs in achieving low-power electronics. The data was collected from 15 devices.
Table 1 Comparison of flexible transistor performance of SGTs and ohmic TFTs
  SS (mV dec−1) Off current (A) On/off ratio V th (V)
SGT 164.73 ± s37.01 ∼10−8 2.2 × 105 0.42 ± 0.35
Ohmic TFT 191.31 ± 72.93 ∼10−7 8.2 × 103 −1.38 ± 0.62


Finally, to evaluate the mechanical flexibility and durability of flexible SGTs, we performed electrical tests under varied bending radii and different bending/releasing cycles. Fig. 6a shows the transfer curves of flexible SGTs at different bending radii up to 5 mm, where minor performance degradation was observed. Corresponding variations of SS, Vth, and Ion/Ioff were extracted in Fig. 6b. Still, under bending status (5 mm radius), the SGTs maintained high electrical performance with low SS of 170.4 ± 22.0 mV dec−1, low Vth of 0.76 ± 0.11 V, and a good Ion/Ioff of 1.2 ± 0.1 × 105. To investigate the robustness of the flexible SGTs, the devices were repeatedly bent/unbent with a radius of 15 mm. The SGTs exhibit high mechanical stability, showing a low SS of 178.0 ± 10.9 mV dec−1, minor Vth shift (ΔVth < 0.1 V), and high Ion/Ioff of 4.2 ± 1.6 × 104, even after 400 testing cycles. Such high electrical and mechanical performance enables low-power electronic application targeting portable and wearable interfaces. Further improvements can be achieved by aligning the electronic device layer at the mechanical neutral plane via encapsulation or passivation.


image file: d0nr06177h-f6.tif
Fig. 6 Demonstration of mechanical flexibility of flexible devices. (a) Transfer curves of flexible SGT at different bending radii. (b) Change of SS, Vth and current on/off ratio variations of flexible SGTs at different bending status. Error bars stand for standard deviations from 3 devices. (c) Transfer curves of SGT at different bending cycles. (d) Change of SS, Vth and current on/off ratio of flexible SGTs with different bending cycles. Error bars stand for standard deviations of 3 measurements.

In conclusion, we achieved high-performance flexible SGTs based on solution processed In2O3 by incorporating a Schottky barrier at source electrodes in TFTs. The Schottky barrier was formed by applying asymmetry electrodes of Au and Al at source and drain contacts, respectively. The In2O3 SGTs hold lower operation voltage, faster saturation, higher gain, and larger output impedance, compared to conventional ohmic TFTs. Our results reveal the feasibility of employing source-gated configuration with the Schottky barrier in achieving low-power electronics that are essential for low-cost flexible electronics. To this end, we demonstrate flexible light-weight In2O3 SGTs on ultrathin polymer substrates. Our flexible SGTs exhibit ultralightweight (0.56 mg cm−2), extremely low saturation voltage (0.2 V), low operation voltage (<2 V), and low power consumption (46.3 μW cm−2), opening new opportunities for applications in large-area electronics ranging from active-matrix displays and sensor arrays to flexible circuits.

Experimental section

Device fabrication

Schottky barrier TFTs with In2O3 channels were fabricated with a bottom-gate, top-contact configuration structure on silicon (p++) substrate with 100 nm thick SiO2 layer. The precursor solution of In2O3 (0.1 M) was prepared by dissolving indium nitrate hydrate in 2-methoxyethanol (2-ME). Acetylacetone and ammonium hydroxide solution with the same molar concentration were added as additives. The precursor solution was stirred for 24 hours at room temperature and then filtered through a 0.2 μm syringe filter prior use. Before spin-coating the oxide precursor solution, the substrates were treated by UV-ozone for 10 min to render the surface hydrophilic. The In2O3 channels were deposited and patterned by direct-light-patterning techniques as reported before.26 Typically, indium nitrate hydrate solution was spun on an Si/SiO2 substrate at 3000 rpm for 30 s, followed by a pre-annealing of 1 min at 100 °C. Samples with different In2O3 film thicknesses were prepared by spin-coating the precursor solution repeatedly and annealing at 100 °C for 1 min at each time. Then, DUV (185 nm and 254 nm) irradiation was applied through a shadow mask to initiate the photochemical activation of the In2O3 sol–gel films. The unexposed areas of In2O3 film were etched in a mixed solution of acetic acid and methanol (1[thin space (1/6-em)]:[thin space (1/6-em)]20, v[thin space (1/6-em)]:[thin space (1/6-em)]v), leaving channel patterns. The photoactivated In2O3 channels were annealed at 300 °C for 1 h. Next, Au (50 nm) and Al (50 nm) were sequentially deposited as asymmetric source and drain electrodes, respectively, by thermal evaporation via shadow masks with manual alignment, forming a channel region of 1000 μm/150 μm (W/L).

Flexible device fabrication

For flexible devices, ultrathin (∼1.7 μm thick) polyimide (PI) films were used as substrates. PI solution was spun on glass substrates at 1000 rpm for 60 s, baked at elevated temperatures of 150 °C, 200 °C, and 250 °C, each for 30 min, and finally annealed at 300 °C for 1 hour in a N2 environment. A thin layer of Al2O3 was deposited on the PI film as the buffer layer by spin-coating 0.2 M aluminum nitrate solution in 2-ME and annealing at 300 °C for 2 h. Then 50 nm Al was thermally evaporated as the bottom gate. After UV-ozone treatment, aluminum nitrate solution (0.2 M) was spun at 3000 rpm for 30 s and then pre-annealed at 100 °C for 1 min. This process was repeated for three times to get the sufficient thickness (∼30 nm) and enough insulating properties, and then the Al2O3 layers were baked at 300 °C for 2 hours. The fabrication processes of the active layer and S/D electrodes were the same as for TFTs on Si/SiO2 substrates. The devices on PI could be mechanically exfoliated from the carrier glass.

Characterisation studies

The cross-sectional image of the PI film was obtained with a field emission scanning electron microscope (Zeiss Gemini500). The surface roughness and thicknesses of In2O3 thin films were measured using atomic force microscopy (AFM, Bruker Dimension ICON). The AFM sample for height profile was fabricated using conventional photolithography and subsequent wet etching with dilute hydrochloric acid in water (HCl[thin space (1/6-em)]:[thin space (1/6-em)]H2O = 1[thin space (1/6-em)]:[thin space (1/6-em)]10, v[thin space (1/6-em)]:[thin space (1/6-em)]v). The thickness of the Al2O3 dielectric was measured using an ellipsometer (J.A. Woollam RC2 XI+). The optical transmittance of the films was recorded using a UV-visible spectrophotometer (Shimadzu UV-2700). The ultraviolet photoelectron spectroscopy (UPS) spectra were recorded using He I irradiation with = 21.22 eV at −9 V bias (AXIS Ultra DLD, Kratos).

Electrical characteristics were measured in an ambient environment using a semiconductor parameter analyzer (Keithley 4200A-SCS) and/or source meter (Keysight 2912B) integrated with probe station systems.

The carrier concentration (ND) extraction equation is expressed as

 
image file: d0nr06177h-t2.tif(2)
where V is the voltage bias applied on metal, Vth is the voltage bias applied on metal, and ψ0 is the work function difference between metal and semiconductor.

The parameter extraction equation is expressed as

 
image file: d0nr06177h-t3.tif(3)
 
image file: d0nr06177h-t4.tif(4)
where L and W are the channel length and width of the transistors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge support from the Westlake Multidisciplinary Research Initiative Center (MRIC) (Grant No. MRIC20200101). H. W. acknowledges the support from the National Natural Science Foundation of China (Grant No. 61574107). B. Z. thanks Prof. Yang Yang and Dr Lei Meng for discussion, advice and assistance. This work was performed in part at the Westlake Center for Micro/Nano Fabrication and the Instrumentation and Service Center for Physical Sciences (ISCPS), Westlake University.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr06177h

This journal is © The Royal Society of Chemistry 2020